Yes it would break down but right now I'm specifically looking for neighbouring dupes :) (There is an actual reason to suspect they are positioned so in those files)
It wouldn't be hard for me to write a push/shift code that would kind of like slide a 4-line long reading frame over the whole text file.

But I ran totally in a ditch trying to do the same so that the frame wouldn't "slide" but would be "lifted" four lines at a time.

Then I could just

<code> if ($frame[1]!=m/$frame[3]/){print @frame} <\code>

But for some reason I mess the populating/emptying/and moving the frame up so readily.

edit: disregard all that - a sliding frame is exactly what I need. So I guess were done here :D. Thank you all! Learned a lot of other stuff on the side also :)